Structure and morphology of Mg-Al-Fe-mixed oxides derived from layered double hydroxides.

The influences of the nature and the extent of M(III) ion substitution on the structure, morphology and surface properties of layered double hydroxides, LDHs Mg(1-x) M(III)(x)(OH)(2)(x/n).mH(2)O, M(III) being Al or/and Fe and x= M(III)/[(Mg+M(III)], and derived mixed oxides were investigated. Three series: Mg-Al, Mg-Al-Fe and Mg-Fe were synthesized using low supersaturation co-precipitation method at constant pH, with different Mg : Al : Fe ratio and x in the wide range from 0.15 to 0.7 in order to obtain complex, multi-phase systems with disordered structure, developed surface area, acid-base and redox properties favourable for catalytic application. The morphology of LDHs and their derived mixed oxides did not change considerably although pronounced changes in structural and surface properties occur by thermal decomposition. The increase in Al amount, as well as the deviation of M(III) content from the optimal range for the single LDH phase synthesis, causes the formation of smaller particles and decrease of mixed oxide crystallite size. The nature and amount of M(III) influence the development of surface area, after thermal treatment, depending mainly on the presence of smaller mesopores, not visible by scanning electron microscope. Although the particle size has no considerable influence on the value of the surface area, it was observed that the samples with smaller particles (Mg-Al and Mg-Al-Fe series) have also higher surface area compared with the samples with larger particles (Mg-Fe series).